This invention relates to a method of spray drying a feedstock to produce an agglomerated product, comprising the steps of supplying said feedstock to at least one spraying device arranged in the upper part of a vertically disposed drying chamber, introducing a drying gas into a processing zone of the chamber outside said spraying device, forming a layer of fluidized particles in a lower region of the chamber and exhausting said drying gas from the chamber through outlet means disposed in said upper chamber part.
Whereas the term "spray drying" refers, in general, to processes in which a fluid feedstock is at a relatively low temperature compared to the temperature of the drying gas, the feedstock may also be at a relatively high temperature with respect to the gas in processes which are sometimes referred to as "spray cooling". In the context of the present invention the generic term "spray drying" is used to encompass all such processes in which a fluid feedstock is sprayed into a drying gas.
Spray drying methods involving, in addition to the spraying of a liquid feedstock as small droplets of preferably uniform size distribution into a hot stream of drying gas, the use of a fluid bed integrated in the lower part of the drying chamber to expose the primary spray particles to further drying and classification are well known in the art and serves to produce a free flowing agglomerated product having desirable properties for many types of spray dried products such as dairy, food, pharmaceutical and chemical products, in particular socalled "instant" properties requiring high wettability and solubility and/or with specified properties with respect to flowability and absence of small particles or fines, the presence of which in the product may cause handling problems and less acceptable conditions of occupational health in the performance of such methods.
Typically, these methods also known as the MSD.TM. or FSD.TM. process (Trademarks of NIRO A/S) involves the steps of atomizing a liquid feedstock into a spray of small droplets in the processing zone, where the droplets are contacted with the hot drying gas stream having a high velocity to produce partly dried primary particles, and formation of agglomerates by mixing the primary particles with fine or dustlike particles also known as fines, which are entrained from the drying chamber itself or returned to the agglomeration zones in the chamber from auxiliary equipment such as a cyclone outside the drying chamber, in which fines are retrieved from the exhaust of drying gas from the drying chamber, and subsequent additional drying in the fluid bed also serving to provide cooling of the agglomerated particles.
The main agglomeration zones in the chamber are the spraying zone or nozzle cloud immediately outside the spraying device, which is typically a spraying nozzle, the fluid bed which is typically located in the lower part of the chamber and on the inner surface of a conical part of the chamber, if present.
Typical prior art spray drying devices for the performance of the MSD and FSD methods as described can be found in published International Patent Application WO 95/24599, EP-B-097 484, EP-A-0 749 769 and EP-A-0 749 770 all of which suggest after-treatment of the product obtained from the fluid bed in a further fluid bed either by designing the fluid bed integrated in the drying chamber as a two stage fluid bed or arranging a separate after-treatment fluid bed outside the drying chamber.
In many agglomeration processes the return of fines to the agglomeration zones of the drying chamber plays a significant role for the result to be obtained, and the properties of the agglomerated product will depend on many factors such as the part of the chamber to which fines are returned, such as discussed in EP-A-0 378 498 and WO 95/13864.
As disclosed therein as well as in EP-A-0 749769 conventional prior art suggestions for return of fines for use in an agglomeration process have suffered in general, however, from the disadvantage that the fines to be returned must be collected outside the chamber and transported to the chamber by a sometimes quite complicated ducting system.
Moreover, for a number of products incorporating poisonous, polluting or otherwise dangerous or hazardous chemicals such as a number of pharmaceutical products, dyestuffs etc. it is highly desirable and likely to become an environmental demand to improve safety by avoiding the handling of such materials outside the processing chamber.
In U.S. Pat. No. 4,657,767, DE-A1-195 08 657 and DE-A1-195 11 961 the incorporation of integrated filter elements in dryers is disclosed per se. U.S. Pat. No. 4,657,76 discloses a substantially horizontally disposed processing chamber with the spraying device and an arrangement of bag-type filter elements located at opposite ends of an upper chamber zone, whereby agglomerates formed to some extent on the filter elements are only dislodged to the region underlying the filter arrangement which is outside the main agglomeration zone. Also in DE-A-195 08 657 a horizontally disposed chamber is disclosed in which a filter wall extends throughout the chamber above the processing zone without any precautions to dislodge material retained by the filter. DE-A1-195 11 961 discloses incorporation of filter elements in a direct flow drying chamber, substantially midway between a feeding device arranged in an upper part of the vertically disposed chamber and a fluid bed in the lower part thereof.